1. Field of the Invention
The present invention relates to a braking system for a vehicle, and particularly, to a hydraulic braking system for reducing the brake pedal stroke during the braking operation.
2. Description of the Related Art
In a conventional vehicle braking system, the hydraulic conduit between the hydraulic braking generator (for example, a master cylinder) and the wheel cylinder is made with multiple circuits so that when one circuit becomes inoperative, the braking force is produced by remaining circuits. In general, such conventional dual circuit systems also utilize tandem master cylinders.
In order to reduce the force required to operate the brake pedal during braking, such a hydraulic braking system may be provided with a servo unit, also referred to as a servo or a booster. The servo unit may be powered by the vacuum intake from the intake manifold, for a vacuum booster, or hydraulic pressure, for a hydraulic booster.
According to the use of a hydraulic booster, as shown in Japanese Patent laid open publication No. 59-227552, reduction of the brake pedal stroke may be attempted by applying the output pressure from the booster directly to the wheel cylinders through one circuit. However, the servo unit must be a hydraulic booster. Reduction of the brake pedal stroke can not be accomplished with a hydraulic brake pressure control unit utilizing a vacuum booster and tandem master cylinders. As described in the publication, when the output hydraulic pressure of the booster is applied directly to the wheel cylinder, it becomes necessary to adjust the output pressure of master cylinders of brake circuits in order to properly distribute the braking force of the vehicle. Therefore, since the booster function depends on the operation of the master cylinder, the design and manufacture of this type of braking system is extraordinarily difficult.
A method for preventing output pressure variations in the hydraulic booster during anti-lock control operation is disclosed in Japanese Patent laid open publication No. 62-166150. This publication discloses a regulator for controlling the output pressure of the hydraulic power source in response to the output pressure of the booster or master cylinder, while at the same time, providing a hydraulic changeover value between the changeover valve and the master cylinder. The output pressure of the regulator is applied to the wheel cylinder only during anti-lock control. During normal operation the regulator pressure is diverted from the wheel cylinder.
Although the conventional problem with regard to anti-lock control is solved by the above-mentioned prior art, the introduction of the output pressure of the regulator to the wheel cylinder prevents the reduction of the brake pedal stroke. However, in Japanese Patent laid open publication No. 62-166150, the hydraulic power source and the regulator are both attached to the conventional anti-lock device. This is because of the added function of preventing pressure variation of the master cylinder in anti-lock control operation.
In recent vehicle brake systems, it is required to provide for various needs while keeping basic braking function. In particular, the aforementioned reduction of stroke of the brake pedal is a priority for a class of vehicles which might require swift braking action. However, this is not necessarily required for other types vehicles. When the conventional braking system is designed and manufactured so as to meet all use requirements for each type of vehicle in which it may be used, the manufacturing cost is increased.
Although it has been attempted to make the parts or device for common use in all vehicles, for reducing stroke of the brake pedal as aforementioned prior art, there is no arrangement by which it is attached to an existing basic brake system. Further, even though there is a device for attaching the anti-lock control function, it is not an arrangement that controls the original function of the basic brake system including reduction of pedal stroke.